Depends how accurately you need the timing. The standard 555 configuration timingisn't symmetric, though there are some variants shown in the data sheet that allow youto adjust the duty cycle to exactly 50%. The CMOS version (TLC555, etc.) can beconfigured so the output drives the timing capacitor directly through a resistor, whichgives close to 50% duty cycle. (The data sheet shows a variant for the standardLM555, too, but it isn't as simple.)

Most varieties can source 20 to 200mA (depending on the specific part number) so you'llstill want a transistor or two to switch the actual current to the load.

You can download a datasheet for the LM555 or TLC555 from National Semiconductor(www.National.com) with all the required info.

Any transistor that is rated at sufficient current - perhaps 500ma would be sufficient.

There are a couple likely circuits for the transistors - I'd probably use a PNP as the seriesDC switch (input power to emitter, resistor from emitter to base, load on collector), inwhich case grounding the base through an appropriate resistor will turn on the power.Something in a TO-39 case may work, depending on the saturation voltage. Practicallyany of the PNP transistors in a TO-220 case will probably work, as they are rarely putin that package if they aren't rated at least for 1A @ 30V. Some variant of TIP32 orTIP42 should be fine, they are rated for 5A at 40 to 100V (depending on the last letter).You can bolt a small heatsink to the tab of the transistor if it tends to get warm, butit MUST be insulated from ground.

I'd suggest an NPN transistor in an emitter follower configuration. Relay coil between emitter and ground. Base directly to output of the NE555 and collector directly to the same 12V source that supplies the NE555. No other components needed.

If you are going to use a relay anyway, why not get one with a 50mA coil and drive itdirectly from the 555?

What I like about the PNP circuit is that the output voltage is not limited by the maximumdrive level of the 555 timer. With the NPN, the maximum available output is about 0.6Vbelow the timer peak output (the difference between the base and emitter voltages.)With the PNP the resistor between emitter and base keeps it off even if the 555 outputdrops a volt or two below your supply voltage (depending on the relative resistor values)but the output voltage when on is limited only by the saturation voltage of the transistor.

In fact, in some cases I'd use an NPN between the base of the PNP and ground and letthe 555 drive that base through a resistor - then the only purpose of the 555 is to drivethe lower transistor, and the output voltage of the entire circuit is practically independentof the output drive level of the 555 timer. (Which could be running on a +5V regulator,for example, without any adverse effect.)

If the switched supply is, for example, a battery charger, where the exact voltage dropacross the load is important for proper operation, then a relay may be your best optionas it won't add to the voltage drop, though it may make an annoying clicking if it is nearyour desk, and relays have a finite number of operations. If it is flashing a lamp then it really doesn't make a lot of difference whether it sees 12.3V or 12.1V, though the circuit needs to be able to handle the peak current when the lamp turns on after being cold.

So there are a number of other factors that might influence your choice of a circuit.In any case, I've found it poor economy to simply minimize the parts count: a coupleresistors (and sometimes an extra transistor or two) can make the circuit much morerobust and able to operate over a wider range of input voltages, current demands, etc.We're talking about $0.60 each for the IC and the power transistor from DigiKey, andyou can get a handful of resistors and transistors to improve the circuit for thechange from a $2 bill.

You do have to do a bit of planning - the base drive current to the transistor, forexample, needs to be sufficient to pass the required current, while switching itreliably off at other times. You need to know how much voltage drop your load cantolerate, and potential switching transients (especially with relay coils.) But it isquite easy to design the switching circuit to handle 60V @ 5A and it should runnearly forever in your application.

The thing about the NE555 is that it has a totem pole output. It switches hard to ground and VCC and handles 200mA in either direction. I've even used them for their output circuit when I don't need a timer (disable the timer function and use it for a switch). One 8-pin DIP at 0.45 and available locally is easier that building a similar output using multiple descrete components. In my opinion, in most cases using anything more than an emitter follower is unnecessary and usually just duplicating circuitry that is already contained inside the NE555.

The "Triple Nickel" with Ra at 4.7M and Rb at 680k with a 0.47uF cap yields 0.506636HzI like the Arduino (sparkfun.com) development platform. It uses an Amtel AT Mega PIC.It also uses a 16kHz or 20kHz ref XTAL.This will also build on your C programing skills.

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